During the last several years, we have demonstrated that in several different metabolically stable proteins detectable racemization occurs in all metabolically stable proteins in mammals, then this reaction may contribute to the complex aging mechanism in mammals. Moreover, the extent of racemization in metabolically stable proteins could be used as a "biochronological" tool for assessing the ages of living human beings as well as natural populations of living mammals. To test these proposals, we have conducted a series of analyses of the human ocular lens nucleus. Several age-related changes take place in the proteins present in the lens nucleus, and it appears that some of these changes can be explained by racemization. The extent of aspartic acid racemization in both tooth dentine and the ocular lens nucleus also has been found to provide a reliable estimate of the ages of both humans and other mammals. We propose to continue our investigations of the biological implications of aspartic acid racemization in metabolically stable proteins. The ocular lens research will concentrate on: 1. elucidating the mechanism(s) of accelerated racemization in brunescent cataracts; and 2. examining the relationship between primary protein structure and racemization rates. Aspartyl residues in proteins present in other metabolically stable tissues will also be analyzed to determine their susceptibility to racemization. Finally, studies will be continued on the use of aspartic acid racemization in biochronology, with emphasis on investigations of whether the aspartic acid racemization rate is a constant in various mammalian species.